Gasoline products delivered to service stations for purchase by consumers are typically formed by blending together a number of different gasoline blending stock products. These blending stock products typically comprise liquid products produced by various processing units in a petroleum refinery. The selection and blending of the blending stock products must be performed so that the resulting gasoline blend (a) meets the necessary octane rating (R+M/2) for the blend and (b) has an acceptable Reid Vapor Pressure (RVP).
The maximum allowable RVP for a gasoline blend varies throughout the year and can also vary geographically. Generally, the maximum allowable RVP for a gasoline blend is much higher during cold weather months than in warm weather conditions. For summer blends, there are federal, state, and/or municipal regulations in place which limit the maximum allowable RVP of the blend in order to reduce gasoline vapor emissions, reduce ozone production levels, and/or alleviate smog conditions. These regulatory restrictions typically extend from about the first of May to the middle of September and commonly require that the RVP of the blend not exceed a maximum value somewhere in the range of from about 6.6 psi to about 9.0 psi.
One component commonly used in forming gasoline blends is butane. Butane has a desirably high octane rating (R+M/2) which averages about 92-93 but also has a high average RVP of about 52 psi. Consequently, butane blending is typically reduced significantly during warm weather months in order to comply with the regulatory restrictions on allowable gasoline RVP.
Moreover, during any time of year, and particularly during cold weather months, the amount of butane or other volatile components contained in a gasoline blend will commonly be lower than is necessary such that the actual RVP of the blend will be well below the maximum RVP limit. The difference between the maximum RVP target of the gasoline blend and the actual vapor pressure of the blend is referred to as the “available RVP margin.” By way of example, the actual RVP of a given gasoline blend might be significantly below the maximum RVP limit because (a) the amount of butane or other volatile components initially included in the blend by the blending operator was set at an unnecessarily low level in order to provide an excessive margin of safety, (b) the blend is delivered to an area having a higher allowable RVP (c) the blend is delivered during a seasonal transition period, and/or (d) the upstream blending system was unable to provide an adequate degree of precision and control to achieve the RVP target.
Different techniques have been used heretofore for adding supplemental butane to gasoline blends having significant available RVP margins. The addition of butane to a gasoline blend having a significant available RVP margin is beneficial to both suppliers and consumers. For suppliers, the addition of butane to a gasoline blend allows the supplier to increase the price margin on the butane by selling the product as gasoline. For the consumer, butane blending is beneficial, particularly during cold weather months, because it increases the overall gasoline supply, reduces the cost of gasoline, and provides better cold weather ignition.
The systems used heretofore for the in-line blending of butane with gasoline streams in pipelines and elsewhere have had significant shortcomings. One type of system previously suggested for in-line butane blending is a feed-forward control system of the type described in U.S. Pat. No. 7,631,671. In the feed-forward blending system, the vapor pressure of the incoming gasoline stream is periodically determined, and the vapor pressure of the incoming butane stream is also periodically determined or a theoretical value for the butane is assigned, and these values are used to predictively calculate and implement a blend ratio based upon an allowable vapor pressure for the blend.
Unfortunately, feed-forward systems of this type have significant shortcomings and disadvantages which adversely affect the operation, performance, and efficiency of the blending system. By way of example, some of the problems and shortcomings experienced with the feed-forward systems include:                1. A high potential for error caused by the need to integrate and rely upon multiple meter and analyzer inputs;        2. The cost of purchasing, installing and maintaining all of the metering systems and analyzers;        3. Difficulty in troubleshooting due to all of the multiple inputs and potential sources of problems and error;        4. A further lack of accuracy, precision, and certainty due to the fact that the individual variabilities of the various inputs can be additive;        5. The necessity of assuming that all of the individual variabilities are additive, thus requiring that a much wider margin of error be accounted for when setting the target RVP (control limit) for blending versus the actual allowable maximum RVP specification for the gasoline, which in turn leads to significant under-blending of butane.        6. A lack of accuracy sufficient to allow truly effective use of the feed-forward system for simultaneously blending-up tank heels; and        7. An inability to adapt to conditions and fluctuations commonly experienced when other components such as W grade natural gasoline or transmix are also concurrently added without online analysis to the gasoline, thus resulting in a significant potential for over-blending or under-blending.        
Also, feed-forward systems require significant oversight of the butane supply to ensure that tight specifications for allowable butane pressure are met. Because a forwardly imposed (calculated) RVP of the downstream blend, rather than an actual measured RVP of the blend, is used to determine the butane injection rate, the minimum variability of the feed-forward system typically will not be less than the variability of the RVP of the butane supply, which may be in the range of ±20%.
Other types of in-line butane blending systems used heretofore have employed feed-back controls wherein RVP determinations for the finished gasoline blend product have been used to calculate the butane blending ratio. Unfortunately, these prior feed-back systems have had significant shortcomings and disadvantages which adversely affect the operation, performance, and efficiency of the blending system. For example, these systems have lacked both the capability and the know-how necessary for consistently hitting and maintaining the target RVP (control limit) for the blend and for adapting to swings in butane RVP or to changes in other feed properties or rates. This, in turn, has resulted in significant under-blending because of the large margin of safety which must be maintained between the target RVP and the maximum allowable RVP specification for the gasoline in order to account for the lack of precision and control. In addition, these same deficiencies have (a) prevented the prior systems from being used for blending-up tank heels or blending concurrently with the injection of W grade natural gasoline, transmix, or other additives, and/or (b) resulted in significant under-blending when attempting to perform such operations in order to maintain an adequate margin of safety.
As is thus apparent, a need exists for an improved in-line butane blending system which will (a) provide greater blending accuracy and efficiency, (b) further reduce missed RVP margins to maximize butane blending, (c) adapt more quickly to pipeline flows, batch changes, and vapor pressures, (d) provide expanded control and blending ranges, (e) provide tighter control of the blending process, (f) eliminate the need for multiple inputs which leads to additive variabilities, (g) provide not only an effective apparatus, but also effective processes for simultaneously blending-up tank heels and accommodating and adapting to significant fluctuations in the injection of other blend components, and/or (h) significantly decrease or eliminate the impact of butane supply pressure changes and variability.